The present invention relates to a multiplexer/demultiplexer for wavelength division multiplexed (WDM, hereinafter) optical signals, and especially to a multiplexer/demultiplexer for WDM optical signals in which the fluctuation of an insertion loss caused by the fluctuation of a wavelength of an optical signal is slight and the WDM optical signals are multiplexed or demultiplexed with high stability.
In the field of the WDM optical communication, it is universally admitted that an arrayed waveguide diffraction grating is promising as a multiplexer/demultiplexer for multiplexing or demultiplexing the WDM optical signals, and various proposals have been made on this subject (Japanese Patent Application Laid-Open Nos.4-116607, 4-1634064, 4-220624, 4-3263084, and 5-157920). Especially, since the arrayed waveguide diffraction grating having a flat passband characteristic hardly shows a variation in an insertion loss caused by the fluctuation of a wavelength of the optical signal and multiplexes or demultiplexes the optical signals with high stability, the engineers in this field place their hopes on this device as one of key technologies in the WDM optical communication (U.S. Pat. No. 5,412,744).
FIG. 1 schematically shows a conventional multiplexer/demultiplexer of an arrayed waveguide diffraction grating type. In this example, nine optical signals having the wavelengths of xcex1 to xcex9 (xcex1 less than xcex2 less than  . . .  less than xcex8 less than xcex9) are multiplexed or demultiplexed by the device shown in FIG. 1. For simplicity of explanation, an optical signal having a wavelength xcexi (i=1, 2, . . . 8, 9) will be expressed by the optical signal xcexi.
As shown in FIG. 1, the conventional multiplexer/demultiplexer for the WDM optical signals are composed of a substrate 201, an input waveguide 202, an input slab waveguide 204, an arrayed waveguide diffraction grating 206 comprising plural channel waveguides 205 having different lengths, an output slab waveguide 207, and nine output waveguides 208. The length of the channel waveguide 205 monotonously increases as the position thereof in the arrayed waveguide diffraction grating 206 is high, and the difference in the length between the adjacent channel waveguides 205 is xcex94L, which will be explained afterward.
FIGS. 2A to 2C schematically shows electric field distributions of the optical signal at important portions in the conventional multiplexer/demultiplexer of the arrayed waveguide diffraction grating type. FIG. 2A shows the electric field distribution 209 of the optical signal at a mode conversion portion 203 in the E-Exe2x80x2 direction, FIG. 2B shows the electric field distribution 211 of the optical signal at the input end 210 of the arrayed waveguide diffraction grating 206 in the F-Fxe2x80x2 direction, and FIG. 2C shows the electric filed distribution 213 at the output end 212 of the arrayed waveguide diffraction grating 206 in the G-Gxe2x80x2 direction.
FIGS. 3A to 3C schematically show phase distributions of the optical signals at the important portions of the conventional multiplexer/demultiplexer of the arrayed waveguide diffraction grating. FIGS. 3A to 3C show phase distributions of the optical signals at the output end 212 of the arrayed waveguide diffraction grating 206 in the G-Gxe2x80x2 direction. FIG. 3A, FIG. 3B and FIG. 3C respectively show the phase distributions 214, 215 and 216 of the optical signals xcex1, xcex5 and xcex9.
FIGS. 4A to 4B schematically show differences in the phase distribution between the optical signals at the output end 212 of the arrayed waveguide diffraction grating 206 in the G-Gxe2x80x2 direction. FIG. 4A show a difference between phase distributions 214 and 215, those respectively corresponding to the optical signals xcex1 and xcex5. FIG. 4B shows the difference between phase distribution 216 and 215, those respectively corresponding to the optical signals xcex9 and xcex5.
FIG. 5 respectively show the electric field distributions 220, 221 and 222 of the optical signals xcex1, xcex5 and xcex9 on a focusing surface 219 of the conventional multiplexer/demultiplexer of the arrayed waveguide diffraction grating type in the H-Hxe2x80x2 direction.
Thereafter, the function of the conventional multiplexer/demultiplexer of the arrayed waveguide diffraction grating type will be explained mainly referring to FIG. 1. The difference in the length xcex94L between the adjacent channel waveguides 205 which constitute the arrayed waveguide diffraction grating 206 is given by the following relation.
xcex94L=2mxcfx80/xcex2(xcex5)xe2x80x83xe2x80x83(1)
In the equation (1), m is a diffraction order number (a positive integer) and xcex2(xcex5) is a propagation constant of the channel waveguide 205 for the optical signal xcex5.
The optical signals xcex1 to xcex9 supplied to the input waveguide 202 successively propagate through the mode conversion portion 203, the input slab waveguide 204, the arrayed waveguide diffraction grating 206, the output slab waveguide 207 and the output waveguides 208.
As shown in FIG. 2A, the electric field distribution 209 of each optical signal at the mode conversion portion 203 shows a symmetric twin-peak-shaped profile in the E-Exe2x80x2 direction.
As shown in FIG. 2B, the electric field distribution 211 of the optical signal at the input end 210 of the arrayed waveguide diffraction grating 206 shows a maximum value and minimum values in the F-Fxe2x80x2 direction because of an effect of diffraction. At the input end 210 of the arrayed waveguide diffraction grating 206, the optical signal is divided, supplied to the respective channel waveguides 205 and propagate therethrough.
As shown in FIG. 2C, the electric field distribution 213 of each of the optical signals xcex1 to xcex9 at the output end 212 of the arrayed waveguide diffraction grating 206 in the G-Gxe2x80x2 direction is a reproduction of the electric field distribution 211 at the input end 210 of the arrayed waveguide diffraction grating 206 in the F-Fxe2x80x2 direction.
As shown in FIGS. 3A to 3C, the phase distributions of the optical signals xcex1, xcex5 and xcex9 at the output end 212 of the arrayed waveguide diffraction grating 206 in the G-Gxe2x80x2 direction are different dependently on the wavelengths of the optical signals. Since the optical signal xcex5 satisfies the equation (1), the phase istribution 215 is symmetric. The phase distributions of the ther optical signals at the output end 212 of the arrayed waveguide diffraction grating 206 incline to the G-Gxe2x80x2 direction in accordance with their propagation constants.
As shown in FIGS. 4A to 4B, the difference in the phase destruction between the optical signals xcex1 and xcex5 and the same between the optical signals xcex9 and xcex5 continuously vary at the output end 212 of the arrayed waveguide diffraction grating 206 in the G-Gxe2x80x2 direction. In the output slab waveguide 207, the respective optical signals propagate in the directions corresponding to these inclinations. Accordingly, the optical signals are respectively focussed at different points Y1 to Y9 (not shown) on a focussing surface 219 of the output slab waveguide 207.
As shown in FIG. 5, the electric filed distributions 220, 221 and 222 on the focusing surface 219 in the H-Hxe2x80x2 direction are affected by aberration of the output slab waveguide 207. The electric field distribution 221 of the optical signal xcex5 reproduces the electric field distribution 209 in the mode conversion portion 203 and shows a symmetric twin-peak-shaped profile. On the other hand, the electric field distributions 220 and 222 of the optical signals xcex1 and xcex9 show asymmetric profiles. Since the main cause of asymmetry is aberration, asymmetry becomes noticeable as the optical signal is focussed near the edges of the focussing surface 219 close-by the arc HHxe2x80x2. The respective optical signals starting from the focussing surface 219 close-by the arc HHxe2x80x2 launch the output waveguides 208, propagate therethrough, and are separately taken out from the terminal end 223.
FIG. 6 schematically shows the relations between the insertion losses 224, 225, 226 and 227 and the wavelength of the conventional multiplexer/demultiplexer of the arrayed waveguide diffraction grating type.
As shown in FIGS. 5 to 6, the insertion losses of the output waveguides 208 are determined by multiplexed integrals of the electric field distributions 220, 221 and 222 of the optical signals on the focusing surface 219 and inherent mode functions of the output waveguides 208. The electric field distribution of the optical signal is displaced on the focusing surface 219 in the H-Hxe2x80x2 direction in accordance with the wavelengths thereof. The profile of the electric field distribution becomes asymmetric as the optical signal is focused at a point which is remote from Y5. When the optical signal is focused around Y5, the profile of the electric field distribution is nearly symmetric, and thereby the insertion loss does not fluctuate so much by a slight fluctuation of the wavelength of the optical signal. On the other hand, when the optical signal is focused near Y1 or Y9, the profile of the electric field distribution 220 or 222 is asymmetric, and thereby the insertion loss noticeably fluctuates by the slight fluctuation of the wavelength of the optical signal. As seen from the relations between the insertion losses of the output waveguides 208 and the wavelength shown in FIG. 6, although the passband characteristic 227 of the optical signal xcex5 is flat, those of the optical signals xcex1 and xcex9, both respectively corresponding to the insertion loss against the wavelength characteristics 225 and 226, are inclined.
As mentioned in the above, according to the conventional multiplexer/demultiplexer of the arrayed waveguide diffraction grating type, since the passband characteristic of the insertion loss is not flat because of aberration of the output slab waveguide, the insertion loss significantly fluctuates by the fluctuation of the wavelength of the optical signal, and the characteristic thereof is not sufficiently satisfactory.
Accordingly, it is an object of the invention to provide a multiplexer/demultiplexer in which an insertion loss of an optical signal does not significantly fluctuate by a fluctuation of a wavelength of an optical signal and optical signals are multiplexed or demultiplexed with high stability.
According to the feature of the invention, an multiplexer/demultiplexer comprises:
at least one input waveguide formed on a substrate,
plural output waveguides formed on the substrate,
an arrayed waveguide diffraction grating composed of plural channel waveguides successively extending in a length by xcex94L,
an input slab waveguide for coupling the at least one input waveguide with an input end of the arrayed waveguide diffraction grating, and
an output slab waveguide for coupling an output end of the arrayed waveguide diffraction grating with the output waveguides,
wherein an interval between central axes of the adjacent channel waveguides composing the arrayed waveguide diffraction grating slowly varies through the whole channel waveguides at one or both of the input and output ends of the arrayed waveguide diffraction grating.
In the multiplexer/demultiplexer for the WDM optical signals, it is desirable that the central axes of the channel waveguides forming the arrayed waveguide diffraction grating are radially disposed with respect to the first and second predetermined reference points at both the input and output ends of the arrayed waveguide diffraction grating, and angles formed by the central axes of the adjacent channel waveguides slowly vary through the whole channel waveguides.